Everyone casually throws the term "virus" nowadays, but do we know what a virus is? Considering the recent COVID-19 outbreak that had us all on lockdown in our houses for a whole year, I feel it's crucial for us to really understand what a virus is, how it evolves, and how it spreads from person to person.
What is a Virus?
A virus is a microscopic, infectious agent of simple composition that can only multiply in living life forms. Viruses can infect all life forms, from animals to microorganisms, in order for them to survive and reproduce. However, the classification of a virus is a bit complicated. Viruses aren't plants, animals, or prokaryotic cells; they actually have their own classification in their own kingdom because of their uniqueness. In fact, viruses aren't even living organisms since they aren't free-living (they cannot reproduce without a host cell).
All viruses must have nucleic acids and a protein to be considered a virus. The nucleic acid encodes all the information for each virus, and at least one protein in a virus constructs a shell called a capsid around the virus. The capsid has three main functions: protect the nucleic acid from digestion, provide sites to latch onto the receptors of the host cell, and provide proteins that allow the cell to penetrate through the cell membrane of the host organism so the infectious nucleic acid can get into the interior of the host cell.
Since they cannot generate or store ATP (energy in the form of adenosine triphosphate), they are energy parasites. They latch onto a host cell and utilize their ATP in the host cell to which it has attached itself. They are also quintessential parasites. This means that they rely on the host cell for almost all of their life-sustaining functions (e.g., ribosomes).
It's also important to remember that viruses have specific conditions for their host cells. A virus cannot simply choose a random host cell; the host cell has to meet certain requirements for that virus to latch onto it.
How Do They Replicate?
Viruses can replicate using two viral replication cycles: the lytic and lysogenic cycles.
In the lytic cycle, viruses typically bind to a receptor on the host cell, allowing it to attach to the outside of the host cell. Then, the virus injects its DNA/RNA into the cell. Sometimes, the virus itself goes inside the host cell without it noticing. Once the genetic material is in the cell, the host cell begins following the instructions for the genetic material the virus has implanted. The instructions tell the host cell to make copies of the virus using its own resources. Once the host cell has created too many copies of the bacteria, it causes the host cell's cell membrane to rupture, letting out all the bacteria inside the cell. Then, those bacteria can spread to other potential host cells and start this process again. The rupturing of the cell membrane essentially kills off the host cell, which is bad when the bacteria continue this process for every other potential host cell it can find.
The other cycle is the lysogenic cycle, which is very similar to the early lytic cycle. The virus begins by injecting its genetic material into the cell, except this time, it's hidden in the host cell's genetic material. So, when the host cell replicates, the new cell also has the viral genetic material. When those cells split again, the daughter cells will also have the viral genetic material. While it may not seem like a problem, it becomes very dangerous if the viruses go into the lytic cycle once they've duplicated through the lysogenic cycle. All those cells with the viral genetic material will start making viruses, which is much more than if they just used the lytic cycle.
How Do They Evolve?
As mentioned previously in my post about evolution and natural selection, mutations happen frequently in a population. When viruses replicate themselves through either the lytic or lysogenic cycle, they will have mutations in some replicas. Any changes to the host or its environment don't affect the virus whatsoever because of its high mutation rate. This is why it's so difficult to find a cure for many types of diseases. For example, if we had a cure for a disease, by the time the cure gets passed around, it's likely that there is a mutation of that disease that is now immune to this cure.
Where Do Viruses Come From?
There are three hypotheses:
Regressive Hypothesis: This hypothesis suggests that viruses were once small cells that parasited off of larger cells and that, over time, the genes that weren't associated with the parasitism were lost.
Cellular Origin Hypothesis: This hypothesis suggests that viruses evolved from bits of DNA/RNA that escaped from the genes of an organism. The bits of DNA may have become plasmids/transposons that would later evolve into viruses.
Co-Evolution Hypothesis: This hypothesis suggests that viruses evolved from complex proteins and nucleic acids when cells first appeared on Earth, making them dependent on cellular hosts for billions of years.
...So Is There Anything Positive?
Yes! Viruses in the modern world can actually be used for gene therapy. Engineers have created this thing called a bacteriophage that acts like a virus and injects genetic material into the cell of a host. However, what's different about a bacteriophage is that you can pick what genetic information you want inserted in the host. This can help the host remove an infectious disease, carry therapeutic genes into the cells, restore protein errors, and correct DNA errors. Additionally, since viruses can go after organisms like bugs, they could also be potentially used as a pesticide. So, despite all the negatives of viruses, there is still a silver lining!
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